The present invention relates to farnesoid X receptors (FXR). More particularly, the present invention relates to compounds useful as agonists for FXR, pharmaceutical formulations comprising such compounds, and therapeutic use of the same.
FXR is a member of the nuclear receptor class of ligand-activated transcription factors. Physiological concentrations of bile acids bind and activate FXR. [Parks, D. J., et al. 1999 Science 284:1365-1368; Makishima, M., et al. 1999 Science 284:1362-1365] Bile acids are amphipathic molecules that form micelles and emulsify dietary lipids. This property also makes bile acids cytotoxic if sufficient concentrations are achieved and thus mechanisms have evolved to ensure bile acid concentrations are tightly regulated. FXR plays a key role in regulating bile acid homeostasis. [Makishima, M. 2005 J Pharmacol Sci 97:177-183; Kuipers, F. et al. 2004 Rev Endocrine Metab Disorders 5:319-326]
FXR is expressed in liver, intestine, kidney, and adrenal. [Kuipers, F. et al. 2004 Rev Endocrine Metab Disorders 5:319-326] FXR target genes in hepatocytes include SHP which encodes small heterodimer partner, an atypical nuclear receptor that represses transcription of genes such as CYP7A1 (encoding cholesterol 7-hydroxylase), the first and rate limiting step in the conversion of cholesterol to bile acid, CYP8B1 (encoding sterol 12-hydroxylase) which controls the hydrophobicity of the bile pool and NTCP (encoding the sodium/taurocholate co-transporting polypeptide) that imports bile acids from the portal and systemic circulation into the hepatocyte. [Goodwin, B., et al. 2000 Mol. Cell. 6:517-526; del Castillo-Olivares, A., et al 2001 Nucleic Acids Res. 29:4035-4042; Denson, L A, et al. 2001 Gastroenterology 121(1):140-147] Other FXR target genes that are induced in liver include the canalicular transporter BSEP (encoding the bile salt export pump) that transports bile acids from the hepatocyte into the bile, multi-drug resistance (MDR3) (encoding the canalicular phospholipid flippase) that transports phospholipids from the hepatocyte into the bile and MRP2 (encoding multidrug resistance-related protein-2) that transports conjugated bilirubin, glutathione and glutathione conjugates into bile. Ananthanarayanan, M., et al. 2001 J. Biol. Chem. 276:28857-28865; Huang, L et al., 2003 J. Biol. Chem. 278(51):51085-51090; Kast, H. R., et al. 2002 J. Biol. Chem. 277:2908-2915.
In the intestine FXR also induces expression of SHP which represses transcription of the ASBT gene which encodes the high affinity apical sodium dependent bile acid transporter that moves bile acids from the intestinal lumen into the enterocyte as part of the enterohepatic recycling of bile acids. [Li, H., et al. 2005 Am. J. Physiol. Gastrointest. Liver Physiol. 288(1)G60-6] IBABP gene expression is also induced by FXR agonists in the enterocyte. [Grober, J. et al., 1999 J. Biol. Chem. 274(42):29749-54] The function of this ileal bile acid binding protein remains under investigation.
Cholestasis is a condition of reduced or arrested bile flow. Unresolved cholestasis leads to liver damage such as that seen in primary biliary cirrhosis (PBC) and primary sclerosing cholangitis (PSC), two cholestatic liver diseases. FXR agonists have been shown to protect the liver in rodent models of cholestatic liver disease. [Liu, Y. et al. 2003 JCI 112(11):1678-1687; Fiorucci, S., et al JPET 313(2):604-612; Pelicciari, R., et al 2002 J. Med. Chem. 45(17):3569-3572]
FXR is also expressed in hepatic stellate cells (HSC) which play a role in deposition of extracellular matrix during the fibrotic process. Treatment of cultured HSCs with the FXR agonist 6-ethyl-chenodeoxycholic acid (6EtCDCA) results in decreased expression of fibrotic markers such as -smooth muscle actin and 1(I)collagen. 6EtCDCA has also been reported to prevent development and promote resolution of hepatic fibrosis in multiple rodent models of this disease. Fiorucci, S., et al. 2004 Gastroenterology 127(5):1497-1512; Fiorucci, S., et al. 2005 JPET 314(2):584-595. According to Fiorucci et al., this anti-fibrotic effect is due to SHP inactivation of Jun and subsequent repression of tissue inhibitor of metalloproteinase 1 (TIMP1) via the AP1 binding site on the TIMP1 promoter.
Recently, S. Kliewer presented data at the Digestive Diseases Week (DDW) Conference (2005) organized by the American Association for the study of Liver Disease (AASLD) showing that activation of FXR by the agonist GW4064 resulted in improved mucosal barrier and decreased bacterial overgrowth in a bile duct-ligated mouse model of cholestasis and intestinal bacterial overgrowth. Dr. Kliewer showed data indicating decreased translocation of bacteria to mesenteric lymph nodes in mice treated with GW4064. This effect of GW4064 was lost in FXR null mice.
The FXR agonist GW4064, when administered to mice on a lithogenic diet, prevented the formation of cholesterol crystals in the bile. This effect of the compound was lost in FXR null mice. Moschetta, A., et al 2004 Nat. Med. 10:1352-1358.